Power Factor Improvement Devices

Power Factor Improvement Devices

For power factor improvement, special steps are taken by using special devices. The devices generally used to improve the power factor are

1. Bank of static capacitors

2. Synchronous condensers

3. Phase advancers. 

1. Static Capacitors

The power factor of the device can be improved by using bank of static capacitors. These capacitors are connected in parallel with the device which is working at lagging power factor. The capacitors draw leading current at an angle of 90° from respective voltage. This helps in partly or completely compensating for the lagging reactive component of load current. Due to this the overall p.f. of the system increases. The p.f. is normally kept at or around unity so as to get p.f. incentive from supply authorities. The capacitor method of power factor improvement finds its applications in most of the industries.

In case of three phase circuits, the capacitor bank connected either in star or delta is used which is shown in the Fig. 7.21.1.

Advantages

a) As capacitors are static devices without having any rotating parts, this method of p.f. improvement is nearly maintenance free.

b) The losses are less in this system.

c) Installation of capacitors is easier as they are light without requirement of any foundation.

d) The working of capacitors is not affected considerably with ordinary atmospheric conditions.

Limitations

a) The repair of capacitors after damage is not economical.

b) The life span of capacitors is short ranging from 8 to 10 years.

c) With the increase in voltage from rated value, capacitors can be easily damaged. 

2. Synchronous Condenser

The over excited synchronous motor takes a leading current and its behaviour is similar to that of a capacitor. If such a motor is operated at no load and connected in parallel with the equipment operating at lagging p.f. then the overall p.f. of the system improves.

Thus an over excited synchronous motor operated on no load is known as synchronous condenser. It helps in improving p.f. by taking leading current from supply and compensates the lagging reactive component of load. The system and corresponding phasor diagram is shown in Fig. 7.21.2.

The three phase load is operating at low lagging p.f. cos ϕ_L which is to be improved. The synchronous condenser takes a leading current I_M at an angle of  ϕ_M from respective voltage. The resultant supply current I_S is phasor sum of I_M and I_L. Current I_L lags behind the voltage. As the phase angle is less than L, cos ϕ is greater than cos ϕ and p.f. of the system improves.

If the motor is ideal without any losses then phase angle ϕ_M will be 90°. But in practice even at no load losses are present so the phase angle is less than 90°.

The synchronous condensers are extensively used at substations for improving p.f. of the system.

Advantages

i) The excitation to the field of motor can be varied which changes the magnitude of current drawn by motor suitably. This helps in obtaining stepless control of power factor. The p.f. improvement with capacitors can be done in steps by switching on the capacitors from capacitor banks so stepless smooth control is not possible by capacitors.

ii) The removal of fault is easy.

iii) The thermal stability of the windings to short circuit currents is high.

Limitations

i) It requires maintenance due to rotating parts which increases cost.

ii) There are significant losses in motor and its operation produces noise.

iii) The synchronous motors are not self starting hence an auxiliary equipment is required.

iv) Upto rating of 500 kVA their cost is greater than that of static capacitors.

3. Phase Advancers

The p.f. of induction motors can be improved by phase advancers. The p.f. of induction motor is low as its stator winding draws an exciting current which is lagging behind the supply voltage by 90°.

The stator winding can be made free from exciting current if the exciting ampere turns are provided from other a.c. source. The phase advancer performs this task by acting as an a.c. exciter. The main motor and phase advancer are mounted on same shaft. The phase advancer is connected in the rotor circuit of the motor. It helps in supplying exciting ampere turns to the rotor circuit at slip frequency as rotor of an induction motor runs with slip speed less than synchronous speed. Thus with excess ampere turns supplied to the motor then required, the induction motor is operated at leading p.f. similar to synchronous motor.

Advantages

i) The lagging kVAR taken by motor are significantly lowered as the exciting ampere turns are provided at slip frequency.

ii) The phase advancers can be used where synchronous condensers cannot be used conveniently.

Limitation

The principal disadvantage of phase advancer is that they are uneconomical for motors rated below 200 HP.

Review Question

1. Explain the various power factor improvement devices.